1. Field of the Invention
The present invention generally relates to an optical scanner and an image forming apparatus, such as a laser printer, a digital copier, a laser facsimile machine, etc., including the optical scanner.
2. Discussion of the Background Art
Tandem image forming apparatuses that form multicolor images are now widely used. Such tandem image forming apparatus includes multiple image carriers, such as photoreceptors, and an optical scanner that directs an optical scanning light (writing light) onto each image carrier so as to form an electrostatic latent image thereon.
On the image carriers, different color images are formed, for example, by developing such electrostatic latent images with different color toners, and these images are transferred from the image carriers and superimposed one on another onto a transfer medium, such as a sheet of paper. Thus, a multicolor image is formed by superimposing different single color images one on another.
In such image forming apparatuses, if relative positions of the images formed on the image carriers are not aligned with each other, the final multicolor image will have color deviation, which means that the superimposed single color images are not aligned with each other in the multicolor image.
Causes of such color deviation include curvature of a scanning line on the image carrier in a main scanning direction, which is hereinafter referred to as a main scanning line. More specifically, the optical scanner is typically distorted slightly due to shape and assembly error of its components, such as optical parts and holders. Further, a motor generates heat during optical scanning, which tends to thermally deform those optical scanner components. Such shape error, assembly error, and thermal deformation of the optical scanner components can cause curvature of the main scanning line.
Such curvature of the main scanning line is further described below with reference to FIG. 1.
FIG. 1 illustrates a drum-shaped photoreceptor serving as an image carrier and main scanning lines directed on a surface thereof, an ideal main scanning line La and curved main scanning lines Lb and Lc.
A center portion of the curved main scanning line Lb projects downstream in a sub-scanning direction, which is a direction in which the surface of the photoreceptor moves. By contrast, a portion of the curved main scanning line Lc projects upstream in the sub-scanning direction. The direction in which the main scanning line curves is unique to each optical scanner because such curvature is caused by a combination of shape error, assembly error, and thermal deformation of the optical scanner components.
Further, the main scanning line on each of the multiple photoreceptors used in the tandem image forming apparatus curves in a direction unique to each image forming apparatus.
If the main scanning lines on the multiple photoreceptors curve differently from each other, relative positions of the respective color images might differ from each other significantly, and accordingly a resultant image is disturbed by color deviation. In order to reduce the color deviation in multicolor images, it is necessary to correct the curvature of the main scanning lines on the respective photoreceptors even when those main scanning lines curve differently from each other.